1. Field of the Invention
The present invention relates to a three-phase brushless motor composed of a plurality of drive coils attached on a stator substrate, a ring shaped drive magnet that opposes the drive coils with a slight gap therebetween and is firmly fixed on a rotor yoke to be rotatable with a rotation axis, the drive magnet having a plurality of magnetized poles, a magnet that is firmly fixed on the rotor yoke and has the different number of magnetized poles from the number of poles in the drive magnet so as to obtain FG pluses or one rotation pulse, and a magnetic sensor that opposes the magnet and is disposed at a position on the stator substrate, the position being free from an influence of magnetic fluxes from the drive coil and the drive magnet.
2. Description of the Related Art
In the past, a three-phase brushless motor has been used in various electrics devices. For example, the motor is often used as a capstan motor of a Video Tape Recorder (VTR) and a motor for a magnetic disc drive.
One example of such a brushless motor is constructed in order to reduce a start time of the motor (See Japanese Patent Application Publication No. H08-322287/1996).
FIG. 1 is a cross section illustrating a related art brushless motor. FIG. 2 is a schematic view illustrating a position of a drive coil and an FG sensor in relation to a drive magnet in the related art brushless motor. FIG. 3 is a view illustrating one example of an MR element pattern applicable to the FG sensor. FIG. 4 is a view illustrating an equivalent circuit of the MR element applicable to the FG sensor. A related art brushless motor 100 illustrated in FIG. 1 is disclosed in Japanese Patent Application Publication No. H08-322287. Referring to the publication, the motor will be briefly described.
As shown in FIG. 1, the related art motor 100 is composed of a stator and a rotor. The motor 100 is configured as a capstan motor.
The stator is composed of a bearing holder 103 that has a pair of bearings 102 fitted in an upper and a lower portion thereof and is attached on a stator substrate 101, a plurality of drive coils 104 that are attached substantially concentrically with a rotation axis 106 passing through the pair of bearings 102 so as to surround the bearing holder 103, and an FG sensor 105 disposed in an outside of one of the drive coils 104 so as to oppose an FG magnet 110 (described later).
On the other hand, the rotor is composed of the rotation axis 106 that passes through the pair of bearings 102 fitted in the bearing holder 103, a cup-shaped rotor yoke 108 that is attached on a bushing 107 fixed on an upper end of the rotation axis 106 so as to be rotatable in unison with the rotation axis 106, a ring-shaped drive magnet 109 that has a plurality of magnetized poles and is fixed along an inner wall surface of the rotor yoke 108, and a ring-shaped FG magnet 110 that has a plurality of magnetized poles, the number of which is different from that of the drive magnet 109, and is fixed along an outer circumferential of the rotor yoke 108 in order to obtain an FG pulse (rotation speed signal). The drive magnet 109 and the FG magnet 110 are rotatable in unison with the rotor yoke 108.
The plurality of the drive coils 104 attached on the stator substrate 101 oppose the ring-shaped drive magnet 109 firmly fixed along the inner wall surface of the rotor yoke 108, maintaining a slight gap therebetween in the vertical direction. A rotational drive force of the brushless motor 100 is produced therebetween.
In this case, the ring-shaped drive magnet 109 firmly fixed along the inner wall surface of the rotor yoke 108 is magnetized in such a way that, for example, eight poles (four pairs of magnetized poles) sector-shaped zones that are disposed at the equal angular intervals so as to surround the rotation axis 106 are alternatingly magnetized to North pole and South pole. In the drive magnet 109, a pair of magnetized poles composed of one North pole and one South pole is disposed at an angle of 360 degrees (2π radian) in electrical angle. The electrical angle is used to represent an angle by defining an angle between a pair of neighboring magnetized poles (North pole and South pole) as 2π radian (rad).
In addition, as illustrated in FIG. 2, the plurality of the drive coils 104 are composed of U-phase, V-phase, and W-phase, each of which is connected to each phase of three-phase electricity. An arrangement pitch angle of two neighboring drive coils 104 arranged next to each other is set as 240 degrees (4π/3 radian) in electrical angle.
The FG magnet 110 firmly fixed along the outer circumferential of the rotor yoke 108 is magnetized so as to create multiple poles. The FG sensor 105 opposes the FG magnet 110. The FG sensor 105 used here is a so-called MR (magnetic resistance) element composed by arranging a material into a pattern shown in FIG. 3, the material changing its resistance upon application of an outer magnetic field. Apparently from the equivalent circuit of the sensor illustrated in FIG. 4, the device outputs a voltage across a terminal P1 and a terminal P2 depending on the intensity of the outer magnetic field when a predetermined voltage is applied between Vcc and GND, because each resistance in the circuit varies in accordance with the intensity of the magnetic field. With the above configuration, a plurality of FG pulses per one rotation are obtained by measuring the voltage that the FG sensor 105 outputs across the terminals P1 and P2 by detecting a magnetic flux originated from the FG. By the way, the reason why the MR element pattern is so complicated as illustrated in FIG. 3 and an electrical potential difference between the terminals P1 and P2 is to be measured is to cancel an influence of magnetic flux from any magnetic sources except for the FG magnet 110 so as not to output a voltage causing therefrom and to reduce an influence caused from a partial abnormality happening in magnetization in the FG magnet.
While the voltage across the terminals P1 and P2 of the FG sensor (MR element) 105 is measured to obtain a plurality of FG pulses, a leakage flux from the drive magnet 109 interlineating the FG sensor 105 is detected so as to be utilized as a start-up signal determining which coil has to be energized when starting up the brushless motor 100. As illustrated in FIG. 2, the FG sensor 105 is arranged at a position away from the center of the U-phase coil by 15 degree+180 degree×N1 (N1: integer) in electrical angle or a position away from the center of the W-phase coil by 75 degree+180 degree×N2 (N2: integer). The FG sensor (MR element) 105 takes a sum of the voltage across the terminals P1 and P2 to detect a position of the drive magnet 109. Therefore, it is found that which one among the drive coils 104 has to be energized when starting-up. As a result, a normal rotation takes place always to obtain counter electromotive force, thereby reducing a start time of the brushless motor 100 as described in the publication.
By the way, in the related art brushless motor 100, although a start-up performance has been improved since the FG sensor 105 is arranged at the position as described above to actively utilize the leakage magnetic flux from the drive magnet 109 interlinkaging the FG sensor 105, an inherent function of the FG sensor 105 is sacrificed.
Namely, the FG sensor 105 is to obtain a plurality of FG pulses per one rotation by detecting magnetic flux from the FG magnet 110. The leakage flux from the drive magnet 109 and the drive coil 104 has an adverse effect on FG pulses as a signal.
The MR element used as the FG sensor 105 is taken a measure to reduce an influence exerted by outer magnetic field. However, polarity and intensity of the leakage flux from the drive magnet 109 and the drive coils 104 change during rotation of the brushless motor 100 although the cycle thereof is longer than that of magnetic flux produced by the FG magnet. In addition, the detection areas inside the MR element (the FG sensor 105) are not just a point but has a width as shown in FIG. 3 and only the areas situated mutually at a different position inside the MR element can serve to cancel the outer magnetic field. Therefore, when the leakage flux changing its intensity greatly directly opposes the MR element (FG sensor 105), the MR element cannot cancel the leakage flux properly, resulting in an occurrence of an error in the FG pulse as an output of the MR element.
Such an error taking place in the FG pulse leads to a disadvantage in that the rotation speed of the brushless motor 100 is not stably controlled.
Therefore, there has been desired a small three-phase brushless motor comprised by arranging a plurality of drive coils attached on a stator substrate and a ring-shaped drive magnet magnetized to create a plurality of magnetized poles, the ring-shaped drive magnet being firmly fixed to a rotor yoke, so as to oppose mutually leaving a slight gap therebetween; by supporting rotatably the rotor yoke integrally with a rotation axis; by firmly fixing a magnet to the rotor yoke, the magnet having the different number of magnetized poles from that of the drive magnet for obtaining an FG pulse or one rotation pulse; and by attaching a magnetic sensor on the stator substrate so as to oppose the magnet; whereby the FG pulse or one rotation pulse is appropriately detected by the magnetic sensor.